Catherization procedures have been refined for a variety of in vivo sensing techniques. In conjunction with imaging means, catheter apparatus may be precisely located to accomplish the special procedure for which the catheter is to be employed.
In nuclear magnetic resonance (NMR) imaging, the spatial density distribution of coherent nuclear magnetization (usually that of hydrogen nuclei) is mapped and the sensitivity of the scanning apparatus to hydrogenous materials forms the basis for recording the progress of the catheter if the latter is substantially more visible in the imaging apparatus than its surroundings. If the structural portions of the catheter are simply more hydrogenous than the tissue surrounding, the catheter is detectable but a limit is placed on the available contrast. Additionally, tissue in the near environs of the catheter may be obscured. Moreover, most catheters comprise functional elements such as light pipes, cutting devices, electrode wires and the like contained within a semi-rigid sheath made of materials chemically inert to body fluids. These functional elements tend to degrade the magnetic resonance imaging and cause a loss of resolution which in turn renders more uncertain the critical location of the catheter terminus. The present invention overcomes the NMR contrast limitation through provision of a localized and controllable magnetic field source at the catheter tip, whereby the location of the tip is unequivocally determined in an NMR image.